Guanine nucleotide exchange factor (GEF) which may activate RAB3A, a GTPase that regulates synaptic vesicle exocytosis. Promotes the exchange of GDP to GTP, converting inactive GDP-bound Rab proteins into their active GTP-bound form. May also activate RAB8A and RAB8B. . This is the function of RAB3IL1 (RAB3A interacting protein like 1, Ensembl gene identifier ENSG00000167994).